Abstract
Purpose
Though, mechanical dental implant and supported prosthesis failures are considered significant, a comprehensive evaluation is lacking. A systematic review analyzing different aspects related to such failures was therefore done.
Methods
- Electronic search was carried out in PubMed/MEDLINE and Cochrane Library for articles published between 1981 and 2021. Articles were selected using predefined criteria. Data extraction was based on mechanical complications associated with dental implants, prosthetic implant failures, survival rate of implants, mechanical failure of implants placed in the maxilla and mandible, and mechanical complications associated with implant supported over dentures. Quality of included studies was assessed. Meta-analysis for heterogenicity testing, publication bias and implant failure assessment was conducted using MedCalc® Statistical Software version 19.7.
Results
- Eighteen retrospective and prospective studies were included following PRISMA guidelines. Mechanical complications were more in the initial 9 years but reduced later. Abutment screw loosening was one of the more common mechanical complications (16.21%). Maxillary implant failure was greater compared to mandibular implant failure with an odds ratio of 4.66 (95% CI -3.21- 6.75). Failure of implant supported overdentures due to mechanical complications were 3% in the fixed effect, and 2.9% in the random effect model (P < 0.05). The overall prevalence of mechanical failure was between 5.6% and 7.7% (P < 0.05)
Conclusion
Mechanical failures of implant and supported prosthesis have similar prevalence to biological and esthetic failures, and therefore need to be given due credence. Identifying specific factors contributing to such failures can help reduce incidence.
Keywords: Implant mechanical complications, Implant mechanical failure, Overdenture mechanical failure, Prosthesis fracture, Screw fracture, Screw loosening
Graphical abstract
1. Introduction
Dental implant is any object or material, such as an alloplastic substance or other tissue, which is partially or completely inserted or grafted into the body for therapeutic, diagnostic, prosthetic, or experimental purposes.1 Dental implants have a more than 2000 year old history, with the modern dental implant era initiated by Dr Goldberg during World War II.2, 3, 4 One of the most important breakthroughs was made by Per-Ingvar Branemark in 1957 by discovering that bone could grow in proximity to titanium without being rejected. This phenomenon was called ‘osseointegration’.5
Endosseous dental implants have been a successful treatment alternative for restoring missing teeth. On the other hand, treatment still fails, evidenced by reports reviewing reasons for implant failure.6-11Implant Failure is defined as failure of an implant to fulfil its purpose (functional or esthetic) because of mechanical or biological reasons, within a range that has been differentiated from “failure” to “complication”. Initially, the term complication was used in literature for reversible conditions that is, those conditions which could be corrected. In recent literature, however, the term failure has been used for both reversible as well as irreversible conditions associated with dental implants.7 In this systematic review, the terms complication and failure have been used interchangeably for convenience, as studies over a long period of time are included.
Esposito et al.7 stated that the reasons for failure of dental implants include biological failures (related to biological processes such as inadequacy in maintaining osseointegration) and mechanical failures (including fractures of implants, coatings, connecting screws, and prostheses).12, 13, 14, 15 A study published in 1999 divided dental implant failures into 7 classes: according to etiology (host factor, surgical failure, implant selection and restorative failure), timing of the failure, condition of failure, responsible personnel, mode of failure, tissues involved and the origin.7 Implant failures have also been classified into four main categories as per Chee W. et al.- 1) loss of integration, 2) positional failures 3) soft tissue defects, and 4) biomechanical failures.6,8,16
Goodacre et al. divided implant complications into 6 categories: surgical, implant loss, bone loss, peri-implant soft tissue, mechanical, and esthetic/phonetic. The authors stated that fourteen mechanical complications have been identified in literature and their incidence ranged from 30% (loss of retention of implant overdenture clip/attachment) to 1% (implant fractures)7,9 Risk of mechanical complications and failures play a major role in implant dentistry because they can lead to increased repair and remake, waste of time, financial resources, and adversely affect the patient's quality of life.8,10 Early detection of implant failure is important as “a failing implant can be saved if it is detected early, whereas a failed implant cannot be saved and must be removed”.7,17, 18, 19, 20
There have been few clinical reviews that have simultaneously evaluated most of the mechanical complications associated with implants. Further, there is a scarcity of studies and reviews on the effect of mechanical complications on survival of prostheses. Such reviews are essential to identify susceptible components and procedures responsible for mechanical failure, addressing which may lead to a better prognosis of dental implants.
The aim of this review, therefore was to systematically interpret literature for information on the following focused questions for dental implants-What are the different mechanical complications and their incidence; how do these compare for the two arches, and the effect of mechanical complications on survival of prosthesis? Analyzing these factors will be helpful towards the treatment/management of specific failures.
2. Materials and methods
The present systematic review is based on the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) checklist.21 This study is registered on the international prospective register of systematic reviews (PROSPERO; CRD42020183898).
2.1. Search strategy
The following bibliographic databases were searched: PubMed/MEDLINE and the Cochrane Library for articles published on the concerned topic within last 40 years. The search strategy included search keywords relating only to or describing implant failure, mechanical complications associated with dental implants, prosthetic implant failure and survival rate of implants. Search terms were the same for both PUBMED and Cochrane databases.
Articles published in Clinical Implant Dentistry and Related Research, Clinical Oral Implant Research, International Journal of Oral and Maxillofacial Implants and The Journal of Prosthetic Dentistry were also manually searched. The searches were re-run just before the final analyses and further studies retrieved for inclusion. The search language was restricted to English only.
2.2. Selection criteria
Randomized controlled trials (RCTs), Non randomized controlled trials, controlled trials, and prospective and retrospective cohort studies with a follow-up time of 4 or more years were eligible to be included. The analysis was performed using the PICO index:(1) population: patients requiring prosthodontic rehabilitation; (2) intervention: osseointegrated implant-supported prosthesis; (3) comparison: [Not Applicable] (4) Outcome: incidence and cause of different mechanical failures in different systems.
The inclusion criteria for study selection were completely and partially edentulous subjects aged 30–80 years, opposing dentition comprising natural teeth, fixed or implant-supported prosthesis, and material for prosthesis fabrication porcelain fused to metal, all ceramic or all metal. Studies on mechanical failure in single implant-supported crowns, implant-supported fixed partial dentures, occurrence of different types of mechanical failure/complications in implants, comparison of mechanical failures in maxillary and mandibular implant-supported prosthesis, and complications with different retention systems in implant-supported overdenture were eligible to be included in the review.
Exclusion criteria included studies on patients with any systemic disease, history of any addiction, or taking medication for these and patients undergoing radiotherapy/chemotherapy. In vitro studies, animal studies, abstracts and case reports were also excluded from this study.
2.3. Validity assessment
Titles and/or abstracts retrieved using the mentioned search strategy as well as those from additional sources were screened independently by two review authors (AV DA) to identify studies that potentially met the inclusion criteria outlined above. The full text of these potentially eligible studies was retrieved and independently assessed for eligibility by two review team members (AV DA). Any disagreement between them over the eligibility of a particular study was resolved through discussion with a third reviewer (PC). Two reviewers (AV SV) independently assessed the methodological quality of the included studies using MINORS (Methodological Index for Non-randomized studies) criteria (Table 1), according to which an ideal global score for non-comparative studies and comparative studies is 16 and 24 respectively. Of the 11 included non-randomized studies,6 were found to be of high quality and 5 to be of medium quality, based on this index.
Table 1.
Methodological quality assessment.
| Methodological items for non-randomized studies (score 0–2) | ||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| S. No. | Study | Additional criteria in the case of comparative study |
Total | |||||||||||
| Q. No. 1 | Q. No. 2 | Q. No. 3 | Q. No. 4 | Q. No. 5 | Q. No. 6 | Q. No. 7 | Q. No. 8 | Q. No. 9 | Q. No. 10 | Q. No. 11 | Q. No.12 | |||
| 1 | Zarb | 2 | 1 | 2 | 2 | 1 | 1 | 2 | 1 | 12 Medium | ||||
| 2 | Zarb | 1 | 1 | 1 | 1 | 1 | 2 | 2 | 1 | 10 Medium | ||||
| 3 | Naret | 1 | 1 | 1 | 2 | 1 | 1 | 1 | 1 | 9 Medium | ||||
| 4 | Tolman | 2 | 2 | 2 | 1 | 2 | 2 | 2 | 1 | 14 High | ||||
| 5 | Haas | 1 | 2 | 2 | 2 | 1 | 1 | 2 | 1 | 12 Medium | ||||
| 6 | Lazzara et al. | 2 | 1 | 2 | 2 | 2 | 2 | 1 | 1 | 13 High | ||||
| 7 | Parein et al. | 2 | 1 | 2 | 2 | 1 | 2 | 2 | 1 | 13 High | ||||
| 8 | Cha et al. | 1 | 1 | 2 | 2 | 2 | 1 | 1 | 1 | 11 Medium | ||||
| 9 | Ferreiroa et al. | 2 | 2 | 2 | 2 | 1 | 2 | 2 | 1 | 1 | 2 | 1 | 2 | 20 High |
| 10 | Meijer et al. | 2 | 1 | 2 | 2 | 1 | 2 | 1 | 1 | 1 | 2 | 2 | 2 | 19 High |
| 11 | Visser et al. | 2 | 2 | 2 | 2 | 1 | 2 | 2 | 1 | 1 | 2 | 1 | 2 | 20 High |
2.4. Main outcome
This systematic review established major causes of mechanical failures of dental implants, with a vision to improve implant success, by preventing and minimizing such failures.
2.5. Measures of effect
Measures of effect for outcome were de-cementation, Screw loosening, Screw fracture, Framework fracture and Implant fracture.
2.6. Data extraction
Variables assessed for data extraction were implant-related mechanical complications, abutment-related mechanical complications, and superstructure-related mechanical complications. A standardized, pre-piloted form was used to extract data from the included studies for evidence synthesis. This included the presence or absence of specific mechanical complications, the prevalence of mechanical complications and cumulative failure rates for implants, abutments, and superstructures. Additional information as extracted from the included studies was the number of subjects examined, mean age of subjects, mean observation time, number of implants restored, implant systems used, restoration design, and study design.
2.7. Risk of bias assessment
Two review authors independently assessed the risk of bias in included studies by considering the following characteristics: adequacy of randomization sequence generation, treatment allocation concealment, blinding, completeness, and selectiveness of outcome data as any other source of bias. Disagreements between the review authors over risk of bias were resolved by discussion, with involvement of third review author where required. Robins tool 1 and 2 were used for the assessment of risk of bias.
2.8. Strategy of data synthesis
A narrative synthesis of the findings was done from the included studies, structured around the type and region of failure, prosthesis type, type of study and implant design.
Meta-analysis was done where possible, considering the range of different outcomes measured across the small number of existing trials, where studies used similar interventions, comparators, and outcome measures.
2.9. Analysis of subgroups
Subgroup analysis was done for occurrence of different types of implant mechanical complications between prospective & retrospective studies, maxillary & mandibular implant-supported prostheses and different retention systems used with implant-supported overdentures.
2.10. Data analysis
Quantitative analysis was conducted using MedCalc® Statistical Software version 19.7 (MedCalc Software Ltd, Ostend, Belgium; https://www.medcalc.org; 2021) for estimating implant failure, with computations including pooled prevalence summary estimate, fixed and random effect model, Confidence Interval, Proportion, Forest and Funnel Plot, Heterogeneity test, Egger's and Begg's test and p values (<0.05 considered significant).
3. Results
Of the 3670 titles emerging from the online search, 110 were screened out, of which 20 were selected for full-text review after reading the abstract. Two studies were excluded as the full text was not retrieved. Out of 18 assessed studies, 3 were not found eligible after validity assessment, therefore a total 15 studies were included. (Fig. 1).
Fig. 1.
Literature search flow.
The data extracted from included publications were grouped under the following heads after screening.
-
1)
Mechanical complications of implant retained prosthesis in prospective studies.
-
2)
Mechanical complications of implant retained prosthesis in retrospective studies.
-
3)
Mechanical failures of Mandibular versus Maxillary implant retained prosthesis.
-
4)
Comparison of different types of mechanical failures
-
5)
Mechanical complications related to implant design
-
6)
Mechanical complications associated with implant-supported overdentures.
There were four prospective studies dealing with mechanical complications of implant retained prosthesis included in this review (Table 2a). Zarb et al. in the year 1990 observed mechanical complications of implant supported prosthesis after placing 274 Branemark system implants in 46 patients. After 9 years, gold screw fracture was found to occur in 19.34%, framework fracture in 4.74% and abutment screw fracture in 3.28% of implant supported prosthesis, with no incidence of implant fracture reported. They used a screw retained retention system for all prosthesis. Naret et al. in the year 1992 also studied mechanical complications in Branemark implant system supported prosthesis with a 7 year follow up period. The Retention system used in the study was also screw retained, with recorded incidence of implant screw fracture to be 1.24%, abutment screw fracture 0.88% and implant fracture 0.53%.
Table 2.
Mechanical complications of implant retained prosthesis.
|
Table-2a Prospective Studies | |||||||||
|---|---|---|---|---|---|---|---|---|---|
| S. No. | Study | Year | Journal | No. of patients | No. of implants | Implant System | Mean observation time | Retention type | Result |
| 1. | G.A.Zarb et al. (Part-III)11 | 1990 | Journal of Prosthetic Dentistry | 46 | 274 | Branemark Implant system | 4–9 yrs | Screw | Damaged abutment screw −3(1.09%) |
| Implant Fracture-0% | |||||||||
| Abutment screw Fracture-9(3.28%) | |||||||||
| Gold screw Fracture-53(19.34%) | |||||||||
| Framework Fracture-13(4.74%) | |||||||||
| 2. | Naret et al.12 | 1992 | Journal of Prosthetic Dentistry | 91 | 589 | Branemark Implant system |
7yrs | Screw | Implant Fracture −3/564 (0.53%) |
| Abutment screw Fracture-5/564 (0.88%) | |||||||||
| Gold screw Fracture-7/564 (1.24%) | |||||||||
| 3. | Haas13 | 1995 | Journal of Prosthetic Dentistry | 71 | 74 | Branemark Implant system |
6yrs | Screw | Abutment screw loosening 12/74 (16.21%) |
| 4. | Zarb et al.11 | 1996 | Journal of American Dental Association | 45 | 259 (23 -maxilla, 233 – mandible) | Branemark Implant system |
9years then 15yrs | Screw | Damaged abutment screw (0–9 yr)- 3 (1.158%) |
| (9–15yr)-0% | |||||||||
| Implant fracture (0–9 yr)- 3 (1.158%) | |||||||||
| (9–15yr)-0% | |||||||||
| Abutment screw Fracture (0–9 yr)- 9 (3.47%) | |||||||||
| (9–15yr)-13 (5.01%) | |||||||||
| Gold screw Fracture (0–9 yr)- 53 | |||||||||
| (20.46%) | |||||||||
| (9–15yr)-17 (6.56%) | |||||||||
| Framework Fracture (0–9 yr)- 13 | |||||||||
| (5.01%) | |||||||||
| (9–15yr)- 3 (1.158%) | |||||||||
|
Table-2b Retrospective Studies | |||||||||
|---|---|---|---|---|---|---|---|---|---|
| S. No. | Study | Year | Journal | No. of patients | No. of implants | Implant system | Mean observation time | Retention system | Result |
| 1. | Lazzara et al.14 | 1996 | Clinical Oral Implant Research | 625 | 1969 | 3i Implant system (Threaded implant Incorporated Machined surface) and cylindrical implant (plasma sprayed) | 5 yrs. | Screw | Gold screw Fracture (11.8%) |
| Abutment screw Fracture (4.3%) | |||||||||
| Framework Fracture (2.2%) | |||||||||
| 2. | Parein et al.15 | 1997 | The Journal of Prosthetic Dentistry | 152 | 392 | Branemark implant System | 6 yrs. | Screw | Implant Fracture-32.3% |
| Gold screw fracture/Abutment screw fracture- 12.9% | |||||||||
| Gold screw loosening/Abutment screw loosening-6.4% | |||||||||
| 3. | Cha et al.17 | 2013 | Journal of Oral Rehabilitation | 120 | 136 | ITI dental implant (micro thread Osseo speed) | 5 yrs. | Screw & Cement | Cumulative failure rate (Mechanical Failures) screw retained −9.7% |
| Cement retained – 6.9% | |||||||||
| 4 | Ferreiroa et al.16 | 2015 | Journal of Clinical Experimental Dentistry | 98 | 80 40 Screw Retained 40 Cement Retained | 1–4 yrs. | Screw & Cement | Screw loosening complications | |
| Screw Retained −20% | |||||||||
| Cement Retained −5% | |||||||||
| Ceramic veneer fracture | |||||||||
| Screw retained restoration-10% | |||||||||
| Cement retained restoration-5% | |||||||||
A prospective study by Haas et al. done in 1995 with a 6 year follow up, found the mechanical complication of abutment screw loosening to be the highest (16.21%). Later in 1996, Zarb et al. followed up 259 Branemark implants up to 15 years and found that mechanical complications were more in the initial 9 years but reduced later on. Implant screw fracture, which was 20.46% at the end of 9 years reduced to 6.56% incidence between the 9th and 15th year. Trends were similar for framework and implant fractures. The only exception was abutment screw fracture which was 3.47% in the first 9 years and increased subsequently to 5.01% of the total complications in the next 6 years (Fig. 2a).represents the summary estimate of the four prospective studies which accounted to 5.6% (95% CI – 4.37–7.06) in the fixed effect model, and 6.5% (95% CI – 1.83–13.82) in the random effect model (P < 0.05). Test of heterogenicity suggested moderate variability, which could be because of wide age and geographic range. No publication bias was seen (Fig. 2b) as both Egger's and Begg's tests had p value greater than 0.05.
Fig. 2.
2A Forest plot for Mechanical Complications of Implant Retained Prosthesis: Prospective Studies
2B Funnel plot for Mechanical Complications of Implant Retained Prosthesis: Prospective Studies
2C Forest plot for Mechanical Complications of Implant Retained Prosthesis: Retrospective Studies
2D Funnel plot for Mechanical Complications of Implant Retained Prosthesis: Retrospective Studies.
Four retrospective study were included in the systematic review as per the inclusion and exclusion criteria(Table 2b). Lazzara et al., in 1996 studied mechanical complications in 1969 implants with a follow up period of 5 years. They found implant screw fracture having the highest incidence (11.8%), whereas framework fracture accounted for only 2.2% of the mechanical complications.
In a retrospective study done by Parein et al., in 1997 with a follow up period of 6 years using Branemark implants, implant fracture was recorded as the most common mechanical complication (32.3%), with abutment screw fracture and abutment screw loosening accounting for 12.9% and 6.4% of complications. Cha et al. in a retrospective 5 year follow up study on 120 subjects using ITI dental implants and both screw and cement retention systems found that cumulative mechanical failure rates in the screw retained system was 9.7% compared to 6.9% in the cement retained system. Ferreiroa et al. evaluated 80 implants for a 4 year follow up, including both screw and cement retained systems. They found that screw loosening in screw retained implants was 20%, compared to 5% in the cement retained system. The summary estimate of these studies accounted to 5.9% (95% CI – 5.03–6.88 in the fixed effect model and 7.7% (95% CI – 3.25–13.88) in the random effect model (P < 0.05, Fig. 2c). Heterogenicity test showed moderate variability. No publication bias was observed using Egger's and Begs test (Fig. 2d).
Four studies obeyed the inclusion and exclusion criteria for comparison of mechanical complications in maxillary versus mandibular implant retained prosthesis (Table 3a). Naret et al. in their study placed 589 implants and found absolute failure rate was 7% for maxillary and 1.7% for mandibular prosthesis, after 7 years of follow up. Then in 1993, Tolman et al. did a 6.5 year follow up to find overall failure rate of 4.9% in Maxillary implant retained prostheses compared to 2.2% in mandibular prostheses. Branemark endosseous implant system was used in both these studies. Lazzara et al. found that maxillary implant retained prostheses failure rate for threaded implants was 6.2% as compared to 4.5% for cylindrical ones, whereas for the mandibular arch failure rate was more for cylindrical implants (4%) as compared to threaded implants (3%). Cha et al. also found the cumulative survival rate for implant retained prostheses in the mandibular arch was12.4%, compared to 5.7% in the maxillary arch.
Table 3.
Mechanical failures of mandibular versus maxillary implant retained prosthesis.
|
Table 3a: Mechanical Failures | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|
| S.No. | Study | Year | Journal | Implant system | No. of Patients | No. of Implants | Duration of Observation | Type of study | Results of mandibular arch | Results of Maxillary arch |
| 1 | Naret et al.12 | 1992 | The Journal of Prosthetic Dentistry | Branemark Implant system | 91 | 589 | 7yrs. | Prospective | Absolute failure rate- 1.7% | Absolute failure rate- 0.7% |
| 2 | Tolman et al. | 1993 | Mayo Clinic Proceedings | Not mentioned | 353 | 1778 | 6.5yrs. | Prospective | Overall Failure rate 2.2% | Overall Failure rate 4.9% |
| 3 | Lazzara et al. | 1996 | Clinical Oral Implant Research | Not mentioned | 625 | 1969 | 5yrs. | Reterospective | Failure rate threaded implant-3% | Failure rate threaded implant-6.2% |
| Cylindrical implant-4% | Cylindrical implant-4.5% | |||||||||
| 4 | Cha et al. | 2013 | Journal of Oral Rehabil-itation | Not mentioned | 120 | 136 | 5yrs. | Reterospective | Cumulative failure rate-12.4% | Cumulative failure rate-5.7% |
|
Table 3b: Metaanalysis | |||||||
|---|---|---|---|---|---|---|---|
| S.No. | Study or Subgroup | Maxillary |
Mandibular |
Weight | Odds ratio | ||
| Event | Total | Event | Total | ||||
| 1 | Lazzara et al. | 27 | 946 | 17 | 1925 | 36.9% | 3.30(1.79,6.08) |
| 2 | Tolman et al. | 59 | 528 | 27 | 1250 | 63.1% | 5.70(3.57,9.10) |
| 3 | Total (95% CI) | 1474 | 3175 | 100% | 4.66(3.21,6.75) | ||
| 4 | Total Events | 86 | 44 | ||||
Heterogeneity. Chi2 = 1.94, df = 1(p = 0.16); I2 = 48%.
Test of Overall Effect: Z = 8.11 (P < 0.00001).
A total of 1474 implants and 3175 implants were assessed in the maxillary and mandibular arches respectively (Table 3b). Eighty six and 44 of these failed in the maxillary and mandibular arch respectively. Maxillary implant failure was greater as compared to mandibular implant failure with an odds Ratio of 4.66 (95% CI -3.21, 6.75, P < 0.05). A wide heterogeneity in the studies was noted (Fig. 3a and b).
Fig. 3.
3A Forest plot for comparison of implant failures between maxillary and mandibular arch
3B Funnel plot for comparison of implant failures between maxillary and mandibular arch
3C Forest plot for failures in implant supported over-dentures.
Eight studies fulfilling the criteria of this review were included(Table 4a). Screw loosening was found to be 20% by Ferreiroa et al. in their 4 year follow up of screw retained prosthesis, while screw loosening of 5% was found in the same study for cement retained prostheses. Abutment screw fractures ranged from 0.008% (Tolman et al.) to 12.9% (Parein et al.). Incidence of abutment screw fracture was found in the range of 3–5%. Gold implant screw fracture was observed by Naret et al. to be 1.2% and Zarb et al. to be 20.5%. Moreover, in Zarb's study this value decreased to 6.6% in the subsequent 6 year follow up study.
Table 4.
Studies comparing different types of mechanical implant failures.
|
Table-4a Different types of Mechanical Implant Failures | ||||||||
|---|---|---|---|---|---|---|---|---|
| S.No. | Study | Screw loosening | Abutment screw Fracture | Gold screw Fracture |
Ceramic veneer Fracture |
Implant Fracture | Framework Prosthesis Fracture |
Damaged abutment screw |
| 1. | Zarb 1990 | Not Evaluated | 3.2% | 19.3% | 0% | 4.7% | 1.1% | |
| 2. | Naret 1992 | Not Evaluated | 0.8% | 1.2% | 0.5% | |||
| 3. | Tolman 1993 | Not Evaluated | Mandibular0.30% Maxillary-0.008% |
Mandible-0.01% Maxilla-0.008% | ||||
| 4. | Lazzara 1996 | Not Evaluated | 4.3% | 11.8% | 2.2% | |||
| 5. | Zarb 1996 | Not Evaluated | 0-9yr- 3.4% 9-15yr-5.0% | 0-9yr-20.5% 9–15yr 6.6% | 0-9yr-5.0% 9-15yr-1.2% | 0-9yr- 1.2% 9–15yr -0% | ||
| 6. | Parein 1997 | 6.4% | 12.9% | 12.9% | 32.3% | |||
| 7. | Ferreiroa 2015 | Cement Retained prosthesis-5% Screw Retained prosthesis-20% | Not Evaluated | Not Evaluated | Cement Retained--5% Screw Retained −10% | Not Evaluated | Not Evaluated | Not Evaluated |
| 8. | Haas et al. | 16.2% | Not Evaluated | Not Evaluated | Not Evaluated | Not Evaluated | Not Evaluated | Not Evaluated |
|
Table-4b Mechanical failures of cylindrical versus threaded implant design | ||||
|---|---|---|---|---|
| S.No. | Lazzara et al., 1996 | Threaded 3.7 mm (3i Implant machined surface) | Cylindrical 3 mm (3i Implant plasma sprayed) | Cylindrical 4 mm (3i Implant plasma sprayed) |
| 1. | Gold screw fracture | 0.6% | 0 | 0.7% |
| 2. | Abutment screw fracture | 0.3% | 0 | 0 |
| 3. | Framework fracture | 0.1% | 0 | 0 |
Fracture of ceramic veneers as a mechanical complication was found to be approximately 5% with cement retained prosthesis and 10% with screw retained prosthesis in Ferreiroa et al.‘s retrospective study with a 4 year follow up. Implant fracture was found to be negligible in most studies, exception being 32.3% implant fracture incidence reported in a retrospective study by Parein et al. Framework or prosthesis fracture was reported as 4.7% by Zarb et al. and 2.2% by Lazzara et al. Zarb et al. further reported in a subsequent study with longer follow up period a reduced framework fracture rate of 1.2%. Abutment damage was uncommon but reported in some studies (1.1% in a prospective study by Zarb et al.). In a study published by the same investigators in 1996 it was found that in the 9–15 year follow up period, abutment damage was reduced to 0%.
Just one study on screw retained prosthesis by Lazzara et al. in year 1996 was included for this comparison (Table 4b). In this study, the 3i implant system was used with threaded implants having machined surfaces and cylindrical implants with plasma sprayed surface. Mechanical complications like implant screw fracture were 0.7% in cylindrical implants with 4 mm diameter, 0.6% in threaded implants with 3.7 mm diameter and 0% in cylindrical implants with 3 mm diameter. Abutment screw and framework fractures were 0.3% and 0.1% for threaded implants while being 0% for cylindrical implants of both diameters.
Five studies fulfilled the review criteria and were included for comparing mechanical complications for different retention systems(Table 5). Naret et al., in 1999, placed 72 implants for fabricating overdentures using three retentive systems i.e. Dolder bar, magnet and ball attachment. After a 5 year follow up, they found mechanical complications were most with magnet retained implant supported overdentures.
Table 5.
Mechanical complications associated with implant supported overdentures.
| S. No. | Study | Implant System | year | Type of study | No. of subjects | No. of implant studied | Mean Observation periods | Implant Overdenture retention mechanism Design | Results |
|---|---|---|---|---|---|---|---|---|---|
| 1. | Naret et al. | Branemark System | 1999 | Prospective | 36 | 72 | 5 yrs. | 12 Dolder bars 12magnets 12 ball | Complications Magnets > ball > bars |
| 2. | Gotfredson et al. | Astratech | 2000 | Prospective | 26 | 52 | 5 yrs. | 11 bars 19 ball | Per year complication-1 for bar 0.6 for ball |
| 3. | Meijer et al. | IMZ, Noblecare, Straumann | 2000 | Computerized balancing | 61 | 122 | 5 yrs. | 61 bars | Fracture and loosening, Most common complications |
| 4. | Timmerman et al. | Straumann | 2004 | Randomized Control Trial | 103 | 274 | 8.3 yrs. | 36 ball 37 bars on 2 implants 37 bars on 4 implants | Complications Ball > Bar |
| 5. | Visser et al. | IMZ | 2005 | Prospective | 60 | 180 | 5 yrs. | 30 bars on 2 implants 30 bars on 4 implants | Complications 2 implants >4 implants |
In a prospective study by Gotfredson et al., in 2000 comparing bar and ball retention system, complications per year were reported to be 1:0.6 for bar vs ball attachments. In the same year, Meijer et al. found fracture and loosening as common complications in a five year follow up study of 122 implants (IMZ, Noblecare, Straumann) with round bar and Ackerman clip as the retentive system.
Timmerman et al. did the longest follow up (8.3 years) of ball and bar (2 and 4 implant supported) retained overdentures and found that stability of implant supported overdentures was less with the ball attachment system. Visser et al. published a study a year later comparing bar retained overdentures on two versus four implants in a 5 year follow up. They found more complications associated with 2 implant retained over-denture systems (Fig. 3c). represents the summary estimate of studies included to review implant supported overdenture failure owing to mechanical complications. These made up to 3.% (95% CI – 1.55–5.44) in the fixed effect model, and 2.9% (95% CI – 0.01–12.24) in the random effect model (P < 0.05). Heterogenicity test revealed moderate variability. Egger's test and Begg's test showed no publication bias.
4. Discussion
The principal objective of the present systematic review and meta analysis was to extract and summarise information from published literature related to mechanical failures/complications of implant supported prostheses. The collected information would be useful to prevent such mechanical failures in future practice. Studies searched included those describing implant failures, mechanical failures of implant, prosthetic implant failures and survival rate of dental implants. Other implant related failures pertaining to surgical, esthetic and bone loss etiologies were excluded. Studies were included to also understand the occurrence of mechanical complications with time and effect on survival of prosthesis.
Four prospective and four retrospective studies dealing with mechanical complication of implant retained prosthesis were included in the study. Zarb et al., in 1990 in a longitudinal study, studied the surgical as well as prosthetic problems associated with implant supported prosthesis.9, 10, 11 They found that the gold screw was the weakest link of the hardware chain and the major site for mechanical failures. This probably indicated occlusal overload as well and efforts were needed to modify occlusal relationship, dietary habits and prosthesis design. They emphasized the need of built-in safety, replaceability and retrievability features in implant systems to meet such challenges.
Naret et al. found the absolute success rate of implant supported prostheses to be 96% and 100% for the maxillae and mandible respectively. Following guidelines were advised to avoid mechanical complications such as abutment screw fracture, framework fracture and abutment loosening, 1) optimum passive fit of framework on top of abutment, 2) screw should be tightened to a maximum preload of 10 Ncm. 3) shorter cantilever to avoid lever effect.12,13 In a study by Haas et al., the most common complication observed in replacements with the single tooth implant was abutment screw loosening, especially in the molar region of the mandible. This problem has decreased since development of the torque controller and gold abutment screw, perhaps because the screw cold-welds to the titanium implant at 32 Ncm.13,14 This study had a high weight in the fixed effect model.
A retrospective study in 1996 found gold screw fractures to be the most common mechanical failure. Though the list of complications reported in this study were similar to the previously described ones, a high degree of success was attributed to factors such as careful patient selection, communication between surgeon and prosthodontist, gentle surgical technique, strict oral hygiene, and regular recall appointments.14,15 Parein et al. in their retrospective study, found that screw-retained single-implant restorations presented significant mechanical complications, such as time dependent metal fatigue whereas cemented single-tooth crowns had minor complications, which occurred more in the first year such as loosening of crown and material fracture.15,16 The authors considered these first year complications to be indicators of factors such as hyper occlusion, excessive function, or para functional activities occurring after insertion.
Ferreiroa et al.‘s study found cemented restorations to prevent loosening problems, but incidence of mucositis and peri-implantitis were more likely. In screw retained restorations, the behaviour seemed exactly the opposite.16,17 Cha et al. in their retrospective study found a high risk of coronal fracture of fixture and screw loosening in the molar area. Coronal fracture of implant had significant association with gender as this occurred earlier in males than females, which could be the result of stronger bite and occlusal forces.17,18 Screw loosening, though one of the more common mechanical complications, was more frequent in screw retained implant prosthesis compared to cement retained counterparts, though the exact cause remains to be established and could be a topic for future studies. Meta analysis of prospective studies placed mechanical complications at 5.6% by the fixed and 6.5% by the random effect model respectively. For retrospective studies this was 5.9% for fixed effect and 7.7% for random effect model, giving a range of 5.6%–7.7% for mechanical complications, signifying these have a major impact in implant prosthesis failures.
Naret et al. observed an absolute success rate of 96% for maxillary and 100% for mandibular implant supported prosthesis. This study observed strict criteria for prosthesis failure and concluded that cumulative failure rate should be mandatory for evaluation of success of the implant retained prosthesis, rather than absolute success rate. There was no significant difference in mechanical failure rates between maxillary and mandibular arches in a study conducted by Cha et al.17 One long term study conducted on implants in posterior region described a 95.5% success rate for the mandible and 95.2% for maxilla.18 A five year study on IMZ (Intramobile cylinder) titanium-plasma sprayed implant reported a success rate of 92.9% in maxilla and 95.8% in mandible.19 In agreement with Kim and Lee the maxilla has a thinner cortical plate and lower bone density than the mandible. Sites with poor bone quality may be at risk of inability to establish primary stability of the implants resulting in early implant failure. High failure rates in the mandibular posterior region have been determined to be caused by overheating of the bone.20
No single study has covered implant mechanical failures comprehensively till date. Abutment screw loosening (more common in the mandibular molar region) and abutment screw fracture were the common mechanical complications observed throughout different implant systems used..13 Studies suggest that the internal hex configuration was less prone to screw loosening when compared to the external hex, because of limited engagement of external member and presence of a shorter fulcrum distance when tipping forces act.23,24 Lazzara et al. found that threaded implants were more prone to mechanical complications.14 Alberketson et al. in a multi-center study using threaded implants, computed success rates of 84.9% for the maxillae and 99.1% for mandible after a follow up of 5–8 years.25
Satisfaction of the patients with retention of overdentures was affected by the attachment mechanism, being best for bar retained devices. The amount of aftercare was higher with spherical attached systems in most reports. Fractures of dentures occurred frequently if no metal frameworks were constructed, especially with bar devices.10Meta analysis revealed Visor et al.‘s study to have the highest weight (50.70%) in the fixed effect model. This study also found a tendency for more biological complications with four implants but a higher need for prosthetic aftercare with two implants.
In a systematic review by Goodacre et al. the mechanical failures associated with implants were found to be less than 10% except for the over denture clip, while esthetic implant failures were reported at 10%. In a study by Esposito et al., biological implant failures were measured to be 7.7% over a duration of 5 years. The findings of this review are in conformance with Goodacre et al.‘s study regarding prevalence of mechanical failure.
In recent trends there is difference in implant material, design, surface modification and so on as compared to implants used in older studies. Factors like type of connection, implant diameter, implant level and bone density also affect the biomechanical nature of implant.29
This review was limited by the difficulty in comparison of studies using different implant systems, follow-up durations and study populations. Also, more comparative studies on threaded and cylindrical dental implants are needed. The single study analyzed in this review found fewer mechanical complications with threaded implants.
The strength of this systematic review with meta-analysis lay in identifying that the mechanical design and integrity of dental implants and supported prostheses need to be given due importance. Components susceptible to failure were identified with design modifications for possible addressal. For patients wearing an implant-supported prosthesis, this can lead to decreased risk of repair and remake, waste of time and financial resources, and adverse effect on quality of life.
5. Conclusion and clinical relevance
The overall prevalence of mechanical failure assessed in the present review was between 5.6% and 7.7% (P < 0.05), and therefore deserve due attention, having a similar prevalence to biological and esthetic complications.The most common mechanical complication was screw loosening followed by screw fracture for implant-supported prosthesis. Maxillary arch was more prone to implant related mechanical complications and failures compared to the mandibular arch, with an odds ratio of 4.66 which was statistically significant. Bar type of retention was found to be most successful followed by ball and magnet-based retention. Meta-analysis revealed mechanical failures of implant-supported overdentures to be between 2.9% and 3%.
Some important points which emerged to reduce or prevent mechanical failures in the present and future are: optimum passive fit of framework on abutment/s, abutment screw tightened to a maximum preload of 10 Ncm, shorter cantilevers, using torque controllers and gold abutment screws, addressing hyper occlusion, excessive function, or para function, opting for internal hex configuration over external hex, and fabricating metal frameworks for implant supported overdentures.
Source of support
Nil.
Declaration of competing interest
Nil.
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